This invention pertains to the field of non-invasive in vivo measurement of blood analytes.
The measurement of blood glucose by diabetic patients has traditionally required the drawing of a blood sample for in vitro analysis. The blood sampling is usually done by the patient himself as a finger puncture, or in the case of a child, by an adult. The need to draw blood for analysis is undesirable for a number of reasons, including discomfort to the patient, resulting in many patients not testing their blood as frequently as recommended, the high cost of glucose testing supplies, and the risk of infection with repeated skin punctures.
Many of the estimated three million Type 1 (juvenile) diabetics in the United States are asked to test their blood glucose six times or more per day in order to adjust their insulin doses for tighter control of their blood glucose. As a result of the discomfort, many of these patients do not test as often as is recommended by their physician, with the consequence of poor blood glucose control. This poor control has been shown to result in increased complications from this disease. Among these complications are blindness, heart disease, kidney disease, ischemic limb disease, and stroke. In addition, there is recent evidence that Type 2 (adult-onset) diabetics (numbering over 10 million in the United States) may reduce the incidence of diabetes-related complications by more tightly controlling their blood glucose. Accordingly, these patients may be asked to test their blood glucose as often as the Type 1 diabetic patients.
It would thus be desirable to obtain fast and reliable measurements of the blood glucose concentration through simple, non-invasive testing. Prior efforts have been unsuccessful in the quest for a sufficiently accurate, non-invasive blood glucose measurement. These attempts have involved the passage of light waves through solid tissues such as the fingertip and the ear lobe and subsequent measurement of the absorption spectra. These efforts have been largely unsuccessful primarily due to the variability of absorption and scatter of the electromagnetic energy in the tissues. Other groups have attempted blood glucose measurement in body fluids such as the anterior chamber, tears, and interstitial fluids. To date, these efforts have not been successful for a variety of reasons.
The present invention combines the accuracy of in vitro laboratory testing of analytes such as blood glucose with the advantages of a rapidly-repeatable non-invasive technology. The invention utilizes a hand-held instrument that allows noninvasive determination of glucose by measurement of the regeneration rate of rhodopsin, the retinal visual pigment, following a light stimulus. The rate of regeneration of rhodopsin is dependent upon the blood glucose concentration, and by measuring the regeneration rate of rhodopsin, blood glucose can be accurately determined. This invention exposes the retina to light of selected wavelengths in selected distributions and subsequently analyzes the reflection from the exposed region.
The rods and cones of the retina are arranged in specific locations in the back of the eye, an anatomical arrangement used in the present invention. The cones, which provide central and color vision, are located with their greatest density in the area of the fovea centralis in the retina. The fovea covers a circular area with a diameter of about 1.5 mm, with a subtended angle of about 3 degrees. The rods are found in the more peripheral portions of the retina and contribute to dim vision.
The light source in the invention that is used to generate the illuminating light is directed on the cones by having the subject look at the light. This naturally provides for the incident light striking the area of the retina where the cones (with their particular rhodopsin) are located. The incoming light preferably subtends an angle much greater than the angle required to include the area of the fovea centralis, so that the entire reflected signal includes the area of high cone density.
The invention uses light that varies in a selected temporal manner, such as a periodically applied stimulus of light (for example, a sinusoidal pattern), and then analyzes the reflected light from the retina to determine the distortion of the detected light relative to the illuminating light. The excitation format chosen allows removal of the light signal due to passive reflection. For example, the primary frequency of an applied sinusoidal stimulus can be filtered out of the light received back from the eye, leaving higher order harmonics of the fundamental as the input into the analysis system (for example, a neural network). Measurement of unknown blood glucose concentration is accomplished by development of a relationship between these input data and corresponding clinically determined blood glucose concentration values.
Similarly, this technique can be utilized in the analysis of photoreactive analytes such as bilirubin. Bilirubin is a molecule that is elevated in a significant number of infants, causing newborn jaundice. It would be desirable to non-invasively measure bilirubin, as this is currently done with invasive blood testing. This molecule absorbs light at 470 nm and exhibits a similar photo-decomposition to rhodopsin, but without regeneration. In a manner similar to that described above for rhodopsin measurement, bilirubin may be measured utilizing a time-varying light signal and analyzing the corresponding reflected light signals for non-passive responses due to photo-decomposition. More generally, an analysisxe2x80x94model-based or statisticalxe2x80x94of descriptors (amplitude, polarization, transient or harmonic content) of incident and detected light can be carried out to determine a variation in the detected signal resulting from light-induced changes in the physical or chemical interaction of a photoreactive analyte with the illuminating light.
In accordance with the invention, a hand-held or stationary instrument that measures the resulting data in the reflected light from a periodically applied light stimulus (for example, a sinusoid) may be utilized for the determination of blood glucose values. There may be patient-to-patient variability and each device may be calibrated for each patient on a regular interval. This may be necessary as the changing state of each patient""s diabetes affects the outer segment metabolism and thus influences the regeneration rates of rhodopsin. The intermittent calibration of the device is useful in patient care as it facilitates the diabetic patient returning to the health-care provider for follow-up of their disease. The device may be equipped with a method of limiting the number of tests, so that follow-up will be required to reactivate the device.
In the present invention, the reflected light data may be sent to a central computer by a communications link in either a wireless or wired manner for central processing of the data. The result may then be sent back to the device for display or be retained to provide a historical record of the individual""s blood glucose levels.
Further objects, features, and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.